1. Field of the Invention
The creation of arable land in dry, desert-like areas such as the Southwestern U.S. has been achieved primarily through the construction of extensive irrigation systems. Streams of water are diverted from a variety of sources to supply a vast network of drainage ditches which direct water to fields, orchards and groves where it is required to sustain the food-producing crops planted there. One unfortunate side effect of the widespread use of irrigation systems is the leaching into the irrigation streams of substances like inorganic salts and heavy metals which can be highly toxic. When the subsoil in any irrigation area includes an impermeable clay layer, excessive irrigation water tends to accumulate if specific provisions are not made for drainage. If the toxic leachate-containing water is not removed, the roots of the irrigated plants will be waterlogged therein and not likely to survive, thereby, resulting in lower agricultural productivity.
However, evaporation and irrigation ponds which have been constructed to improve drainage of toxic leachate-containing water is hazardous to wildlife, as shown by birth defects and high rates of fetal mortality among water fowl which feed and nest in ponds having unacceptably high concentrations of selenium (Se) in the water.
While selenium is a naturally occurring trace element in soil, it becomes highly toxic when concentrated as it has been in recent years in irrigation drainage streams and ponds, and can pose significant dangers to humans and animals if irrigation drainage and evaporation ponds are positioned to allow seepage of selenium-containing drainage water into ground water or other sources of drinking water.
2. Description of the Prior Art
One method of removing selenium as selenate ions from an aqueous solution such as a leaching or mine drainage operation prior to discharge of water into the drinking supply is described in U.S. Pat. No. 4,405,464 to Baldwin et al. This method employs metallic iron in a reduction vessel to reduce the concentration of selenate (Se(VI)) ions to selenite (Se(IV)) ions at a carefully controlled pH, and the ferric hydroxide produced during the reduction must be removed in a separate step. This method, however, does not provide an efficient and effective way to remove selenium from agricultural irrigation drainage systems. Further, this method of removing selenium is too costly for widespread application to an extensive irrigation drainage system, and the method does not contemplate recovery of the removed selenium which is a valuable element.
Activated ferric hydroxide can also be used to remove toxic metals such as selenium from water by a combination of absorption and adsorption, however, this method is costly and too slow to handle the high flow rates likely to be encountered in natural streams and applies only for Se(IV) oxidation state and not for Se(VI).
Methods of removing selenium from non-aqueous systems are also known in the art. For example, U.S. Pat. Nos. 3,876,663 and 3,933,624 to Myers teach removal of selenium from hydrocarbonaceous materials such as synthetic fuels, however, these methods are described to be specifically inapplicable to aqueous solutions.
A method of removing selenium from aqueous solutions containing molybdenum and rhenium is described in U.S. Pat. No. 3,848,069 to Carey et al and employs a series of steps which include a liquid-liquid solvent extraction, which are not suited for detoxification of bodies of water like agricultural drainage streams. Similarly, a precipitation method of removing selenium from acidic waste water like zinc smelter effluent is disclosed in U.S. Pat. No. 3,933,635 to Marchant but the method is not suitable for removing aqueous selenium species from drainage streams.
Other methods for removing selenium from aqueous solution, such as the selenium removal from copper sulfate solutions taught in U.S. Pat. Nos. 4,222,999 and 4,330,508 to Weir et al requires the selenium to be present in solution in the tetravalent oxidation state, and this state may or may not be the case when selenium is found in agricultural drainage streams; therefore, these methods would not be universally applicable for treating all such streams to remove selenium.
Still further methods of removing selenium from smelter gases are known in the art, and U.S. Pat. No. 4,242,124 to Makipirtti is exemplary thereof, however, these methods are not applicable for removal of selenium species from aqueous drainage streams.
Consequently, the prior art lacks a fast, inexpensive and efficient method for removal of selenium from an aqueous stream specifically likely to be encountered from an agricultural irrigation drainage system and which subsequently permits easy recovery of the removed selenium values.
Finally, it is known that bacteria of the genus Clostridium in a closed system under anaerobic conditions or in an environment having a substantial absence of free atmospheric oxygen can reduce the concentration of water soluble selenium ions in aqueous solution; however, the process leaves much to be desired in both the selenium removal rates as well as the length of time required to reduce the amount of selenium in solution. This method is disclosed in U.S. Pat. No. 4,519,913 to Baldwin et al.